Separation of Concerns (SoC) in context of a simple ASP.NET Core WebAPI using C# and Clean Architecture

In the context of a simple ASP.NET Core WebAPI using C# and Clean Architecture, the Separation of Concerns (SoC) principle is applied to ensure that different aspects of the system are organized into distinct and modular components. Here's how SoC is manifested in this scenario:

1. User Interface (UI) Layer:
   • The UI layer in the context of an ASP.NET Core WebAPI is typically represented by controllers that handle HTTP requests and responses.
   • Controllers are responsible for parsing incoming requests, invoking the appropriate application services, and returning the appropriate HTTP responses.
   • They are lightweight and delegate the actual business logic to the Application Layer.

[ApiController]
[Route("api/[controller]")]
public class UserController : ControllerBase
{
    private readonly IUserService _userService;

    public UserController(IUserService userService)
    {
        _userService = userService;
    }

    [HttpPost]
    public IActionResult CreateUser([FromBody] CreateUserRequest request)
    {
        // Delegate to the application layer for user creation
        _userService.CreateUser(request.Username);
        return Ok();
    }

    [HttpGet("{userId}")]
    public IActionResult GetUserById(int userId)
    {
        // Delegate to the application layer for retrieving user by ID
        var username = _userService.GetUserById(userId);
        return Ok(username);
    }
    // Other controller actions...
}

2. Application Layer (High-level Policies):
   • The Application Layer contains application services or use case interactors, which encapsulate the high-level business logic.
   • Application services coordinate the execution of use cases and may involve multiple domain entities and rules.
   • These services are designed to be independent of the external details (e.g., database, framework).

// Interface defining the high-level user service
public interface IUserService
{
    void CreateUser(string username);
    string GetUserById(int userId);
    // Other user-related methods...
}

// High-level user service implementation
public class UserService : IUserService
{
    private readonly IUserRepository _userRepository;

    public UserService(IUserRepository userRepository)
    {
        _userRepository = userRepository;
    }

    public void CreateUser(string username)
    {
        // Business logic for creating a user...
        _userRepository.SaveUser(new User { Username = username });
    }

    public string GetUserById(int userId)
    {
        // Business logic for retrieving a user...
        var user = _userRepository.GetUserById(userId);
        return user?.Username;
    }
    // Other user-related methods...
}

3. Domain Layer (Business Entities and Rules):
   • The Domain Layer encapsulates business entities, domain logic, and business rules.
   • Business entities represent the core data structures and state of the application.
   • Domain logic includes rules and behaviors specific to the business domain.

// Business entity
public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    // Other user-related properties...
}

// Interface defining the low-level user repository
public interface IUserRepository
{
    void SaveUser(User user);
    User GetUserById(int userId);
    // Other data access methods...
}

// Low-level user repository implementation
public class UserRepository : IUserRepository
{
    private readonly List<User> _users = new List<User>();

    public void SaveUser(User user)
    {
        // Database access logic to save the user...
        _users.Add(user);
    }

    public User GetUserById(int userId)
    {
        // Database access logic to retrieve the user...
        return _users.FirstOrDefault(u => u.Id == userId);
    }
    // Other data access methods...
}

4. Infrastructure Layer (External Details):
   • The Infrastructure Layer deals with external concerns like databases, frameworks, and external services.
   • Concrete implementations of interfaces defined in the Application and Domain Layers are provided in this layer.

public void ConfigureServices(IServiceCollection services)
{
    // Register high-level policy module (Application Layer)
    services.AddScoped<IUserService, UserService>();

    // Register low-level detail module (Infrastructure Layer)
    services.AddScoped<IUserRepository, UserRepository>();

    // Other service registrations...
}

By adhering to SoC in Clean Architecture for an ASP.NET Core WebAPI, you achieve a modular and maintainable system. The separation of concerns allows for independent development, testing, and evolution of different components of the system. The UI layer focuses on handling HTTP requests and responses, the Application Layer contains the core business logic, the Domain Layer encapsulates business entities and rules, and the Infrastructure Layer deals with external details. Each layer has its distinct responsibility, promoting a clean and understandable codebase.